As a method of electroreduction of a disulfide compound, a method using an alloy consisting of two or more kinds of particular metals as a cathode is known (patent document 1). In addition, as a process for producing L-cysteine by electroreduction of L-cystine, which is one kind of disulfide compound, a method using a cation exchange membrane as a separating membrane, and an L-cystine solution acidified by adding a mineral acid such as hydrochloric acid and the like to a cathode side electrolytic cell is known (patent document 2). As a process for producing reduced glutathione by electroreduction of oxidized glutathione, a method using an aqueous oxidized glutathione solution acidified by adding a mineral acid such as concentrated hydrochloric acid and the like to a cathode side electrolytic cell is also known (patent document 3).
However, the above-mentioned electroreduction methods of a disulfide compound require use of an expensive electrode. When oxidized glutathione is electroreduced under L-cystine electroreduction conditions described in patent document 2, reduced glutathione cannot be produced in a sufficient yield, since it is unstable under strong acidity and high temperature, as compared to L-cysteine. The process described in patent document 3 also uses, similar to the process described in patent document 2, aqueous oxidized glutathione solution strongly acidified to pH 0.6-1.0 by adding a mineral acid, and therefore, it is associated with the problems of corrosion of cathode and decomposition of reduced glutathione. To minimize the decomposition of reduced glutathione under strong acidity, it is necessary to lower the electric current density. In this case, however, reduction efficiency per electrode area decreases, and therefore, efficient production of reduced glutathione cannot be achieved unless the electrode area is increased instead. That is, production of reduced glutathione in an industrial scale by electroreduction of aqueous oxidized glutathione solution strongly acidified with a mineral acid requires huge electroreduction facility corresponding to the sizes of ion exchange membrane and electrode, as well as a special electrode capable of resisting corrosion. Therefore, a process for producing reduced glutathione by conventional electroreduction is not entirely a realistic method in terms of production efficiency and facility.
To perform electrolysis with good reduction efficiency, which can be practiced on an industrial scale, use of an oxidized glutathione solution with a high concentration is desirable. Patent document 4 describes an electroreduction method utilizing supersaturation of oxidized glutathione, wherein the efficiency of electroreduction is enhanced by this method by using oxidized glutathione at a high concentration, whereby realistic electroreduction in terms of production efficiency and facility can be performed. However, when oxidized glutathione is electroreduced in a supersaturation region, glutathione is rapidly crystallized and precipitated due to the applied voltage, agitation, shock by spray drying, and the like, and the precipitated glutathione crystal may obstruct the electrolytic cell. In addition, since it is an electroreduction in an acidic region, corrosion of cathode is inevitable.
For industrial electroreduction of oxidized glutathione, it is important to continue electroreduction using high concentration oxidized glutathione solution while avoiding corrosion of cathode and without permitting crystallization of oxidized glutathione and reduced glutathione.